There's a difference between labouring for your money and "meaning" money. In the case of celebrities (not just supermodels), while they may also work hard, more importantly they "mean money", mainly through their fame. You could argue that a model just "stands there" or "walks around a little", but if that means customers would buy the product or invest in the business, then in spite of the actual labour put in by the model, the "value" would far outweigh the effort actually made. So it comes down to the people who are willing to pay.

I believe so. I think being a model is a stressful job and it takes a long time to become popular and receive good offers. Beside the average 'lifetime' of a model career is quite short. Of course there are a few 'supermodels' with a long model career earning a lot of money, but in my opinion that's only a small percentage of the number of models in total.

This depends on what services they provide. :D

Some of the most undeserving overpaid people in society.

No, I think that they are overpaid.